Glass and fiber optic plate using the same

ABSTRACT

The object of the present invention is to provide an absorber glass for FOP having a broad absorption band which extends from ultraviolet region through visible region to near infrared region. The absorber glass of the present invention contains FeO, 18 to 40% by weight of SiO 2  and not less than 20% by weight of FeO and Fe 2  O 3  in total.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an absorber glass which can be used ina fiber optic plate (referred to as "FOP" hereinafter) and the FOP usingthe same.

2. Related Background Art

An FOP is a planar image device constituted by a plurality of opticalfibers and used as an optical waveguide for optical instruments such asface plates of image intensifiers and CRTs and CCD couplings.

FIG. 1 is a cross-sectional view of a typical FOP taken along adirection perpendicular to its optical path. As shown in FIG. 1, an FOP500 is constituted by a core glass 502 with a high refractive index, aclad glass 504 with a low refractive index coated on the core glass 502,and an absorber glass 506 for absorbing stray light.

The FOP is manufactured in the following manner. A thin single fiber inwhich a core glass is coated with a clad glass and an absorber glass areassembled together, and thus made assemblies are arranged in a fiberdirection and fused together under high temperature and high pressure.Thereafter, thus fused assemblies are cut in a direction perpendicularto the fiber direction, whereby an FOP having a cross section such asthat shown in FIG. 1 is obtained.

FIG. 2 is a schematic cross-sectional view of the core of the FOP. Asshown in FIG. 2, light incident on the FOP repeats total reflectionwithin the fiber and is efficiently transmitted from the incidentsurface to the exit surface with a high resolution. In this case, partof light leaks from the core glass without being totally reflected. Inorder to prevent such a light component from entering other coreglasses, an absorber glass is disposed. Basic configurations andcharacteristics of the FOP are disclosed in such publications as KazumiNagao, "Optical Fiber," Kyoritsu Shuppan, 1974 and Nakayama et al., ITEJ(the Institute of Television Engineers of Japan) Technical Report, vol.4, no. 53, pp. 1-6, IPU90-44, Sept. 1990.

Conventionally typical absorber glasses contain oxide colorants such asNiO, Ni₂ O₃, CoO, Co₂ O₃, Cr₂ O₃, CuO, MnO₂, SnO, V₂ O₅, and WO₃ forabsorbing stray light. In order to improve characteristics of theabsorber glass, it is necessary to select an oxide colorant having abroad absorption wavelength band. Also, when making FOPs, the oxidecolorant to be selected should not generate discoloring at a fusion stepunder high temperature and high pressure, a drawing step for makingfiber, or the like and should not devitrify the core glass upondiffusion or the like.

Japanese Unexamined Patent Publication No. 2-38343 discloses an absorberglass for FOP which comprises basic five ingredients of SiO₂, B₂ O₃, La₂O₃, BaO, and TiO₂ without including PbO, CdO, Bi₂ O₃, As₂ O₃, Sb₂ O₃, F,and Cl. This publication also mentions the content of each ingredientwhich enables vitrification. For example, it states that, when thecontent of Fe₂ O₃, which is supposed to provide a coloring effect, is15% or more, the glass becomes unstable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an absorber glass forFOP having a broad absorption band which extends from ultraviolet regionthrough visible region to near infrared region.

Another object of the present invention is to provide an absorber glassfor FOP having an absorptivity which is stable and high in a broadabsorption band.

As a result of diligent studies, the inventors have found that a glasscan contain both FeO and Fe₂ O₃ and that the resulting glass has a broadabsorption band which extends from ultraviolet region through visibleregion to near infrared region. Further, as a result of numerous testsand studies based on this finding, the inventors have found that, inorder for the glass to contain both FeO and Fe₂ O₃ at high contents soas to attain a high absorptivity, while enabling vitrification, it iseffective to increase the content of SiO₂ at the same time, therebyaccomplishing the present invention. It has also been found that, whenFeO or Fe₂ O₃ is used alone, its content can be similarly increased whenthe content of SiO₂ is increased.

Accordingly, the absorber glass of the present invention contains 18 to40% by weight of SiO₂ and not smaller than 20% by weight of FeO and Fe₂O₃ in total.

Also, the absorber glass of the present invention may comprise, in % byweight, 18 to 40% of SiO₂, 0 to 10% of B₂ O₃, 0 to 15% of TiO₂, 25 to40% of BaO, 4 to 20% of La₂ O₃, 3 to 30% of FeO, 3 to 30% of Fe₂ O₃, and0 to 15% of Co₂ O₃.

Further, in the absorber glass of the present invention, SiO₂, FeO, andFe₂ O₃, in % by weight, may have the following relationship:

    (content of SiO.sub.2)+(content of FeO)+(content of Fe.sub.2 O.sub.3)>50%

Also, the absorber glass of the present invention may further comprise 3to 6% by weight of Ni₂ O₃.

Further, the absorber glass of the present invention may comprise, in %by weight, 20 to 35% of SiO₂, 0 to 6% of B₂ O₃, 5 to 12% of TiO₂, 25 to35% of BaO, 7 to 10% of La₂ O₃, 3 to 9% of FeO, 7 to 21% of Fe₂ O₃, and0 to 10% of Co₂ O₃.

Also, the absorber glass of the present invention may substantiallycontain neither CoO nor Co₂ O₃.

Further, the absorber glass of the present invention may comprise, in %by weight, 23 to 31% of SiO₂, 0 to 6% of B₂ O₃, 7 to 11% of TiO₂, 30 to32% of BaO, 7 to 8% of La₂ O₃, 3 to 5% of FeO, 8 to 12% of Fe₂ O₃, whilesubstantially containing neither CoO nor Co₂ O₃.

Also, the absorber glass of the present invention may comprise, in % byweight, 28 to 31% of SiO₂, 7 to 11% of TiO₂, 30 to 32% of BaO, 7 to 8%of La₂ O₃, 6 to 9% of FeO, 15 to 21% of Fe₂ O₃, while substantiallycontaining neither CoO nor Co₂ O₃.

Further, the absorber glass of the present invention may substantiallycontain no B₂ O₃.

Since the absorber glass of the present invention contain large amountsof FeO and Fe₂ O₃, it exhibits a high absorptivity in a broad range fromvisible region to near infrared region. As the content of SiO₂ isincreased together, the upper limit of these iron oxides for enablingvitrification is greatly alleviated, whereby a stable glass with a largecontent of the iron oxides can be obtained.

In an embodiment substantially containing neither CoO nor Co₂ O₃, itdoes not contain Co²⁺ and Co³⁺ which are likely to diffuse from theabsorber glass to other parts. Accordingly, when such a glass is used inan FOP, the transmittance of the core is prevented from deterioratingdue to diffusion of cobalt ions into the core. Since this absorber glasscontains FeO and Fe₂ O₃ with neither CoO nor Co₂ O₃, it has a favorableabsorbing capacity in a broad range.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an FOP;

FIG. 2 is a schematic cross-sectional view of a core of an FOP;

FIG. 3 is a graph showing a relationship between transmittance of anabsorber glass and the thickness thereof with respect to a wavelength of790 nm;

FIG. 4 is a spectral transmittance chart for an absorber glass having athickness of 130 μm; and

FIG. 5 is a spectral transmittance chart for an FOP having a thicknessof 3 mm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In recent years, image transmission has been widely performed not onlyin visible region but also in near infrared region. Though theconventional FOPs uses an absorber glass which has a high absorptivityin the visible light near 550 nm, its capacity for absorbing stray lightin near infrared region is not sufficient for preventing the resolutionof images in near infrared region from deteriorating.

While it is indicated that the above-mentioned absorber glass disclosedin Japanese Unexamined Patent Publication No. 2-38343 has a favorableabsorption at 800 nm, no absorptivity is shown with respect to thevisible region centered around the vicinity of 590 nm. Also, it fails tomention any means for increasing the content of colorant oxides so as tomaintain a high absorbing capacity in a broad band.

In the following, the absorber glass of the present invention will beexplained in terms of constituent ingredients thereof.

B₂ O₃

It is an oxide for forming the glass. While vitrification becomes easierwhen this oxide is added to the glass, it is not always necessary forthe glass to contain this ingredient. When its content exceeds 10%, thedensity of the glass becomes so small that the absorbing capacity of theabsorber glass decreases.

SiO₂

It is an oxide, like B₂ O₃, for forming the glass and is essential forthe present invention. When its content is less than 18%, the glasscannot contain a large amount of the colorants mentioned hereinafter.Also, when it exceeds 40%, the content of the colorants cannot beincreased, whereby the absorbing capacity decreases.

TiO₂

When TiO₂ is added to the glass when a large amount of colorants such asFeO and Fe₂ O₃ is contained therein, vitrification becomes easier. Italso functions to harden the glass. When its content exceeds 15%,however, vitrification becomes difficult. Though the effect of additionbecomes small when its content is less than 5%, it may not always becontained, i.e., it may be 0%.

BaO

It functions to broaden the vitrification range and increase thecoefficient of thermal expansion. When a large amount of colorants areadded to the glass in the case where BaO content is less than 5%,vitrification cannot be attained. When it exceeds 40%, on the otherhand, there is no room for colorants to be contained therein.

La₂ O₃

It functions to facilitate vitrification and increase the density of theglass. As the density of the glass increases, the absorbing capacity ofcolorants improves. The effect of addition is small when the content isless than 4%, whereas vitrification becomes difficult when it exceeds20%.

FeO

It is a colorant having a favorable absorption characteristic in nearinfrared region, in particular. When its content is less than 3%, theabsorption characteristic of the absorber glass in near infrared regionbecomes insufficient. When it exceeds 30%, vitrification becomesdifficult.

Fe₂ O₃

It is a colorant having a favorable absorption characteristic fromultraviolet region to visible region, in particular. When its content isless than 3%, the absorption characteristic of the absorber glass innear infrared region becomes insufficient. When it exceeds 30%,vitrification becomes difficult.

Co₂ O₃

Though it is a colorant having an absorption characteristic mainly invisible region, it is not essential for the present invention. When itscontent exceeds 15%, vitrification becomes difficult. As will beexplained later, since Co⁺ diffused into the core portion and therebydeteriorated its transmission characteristic in the FOP prepared with anabsorber glass containing Co₂ O₃, preferably, it may not be contained.

In the following, embodiments of the present invention will beexplained. In the attached drawings, elements identical to each otherwill be referred to with marks identical to each other, without theiroverlapping explanations being repeated.

The method of making the absorber glass fiber in accordance with thepresent invention will be briefly explained in the following. Theingredients in these embodiments had respective material forms asfollows:

    ______________________________________                                        Ingredient      Material                                                      ______________________________________                                        B.sub.2 O.sub.3 boric acid (H.sub.3 BO.sub.3)                                 SiO.sub.2       silicic anhydride (SiO.sub.2)                                 TiO.sub.2       titanium oxide (TiO.sub.2)                                    BaO             barium carbonate (BaCO.sub.3)                                 La.sub.2 O.sub.3                                                                              lanthanum oxide (La.sub.2 O.sub.3)                            FeO             iron (II) oxide (FeO)                                         Fe.sub.2 O.sub.3                                                                              iron (III) oxide (Fe.sub.2 O.sub.3)                           Co.sub.2 O.sub.3                                                                              cobalt (III) oxide (Co.sub.2 O.sub.3)                         ______________________________________                                    

After all these materials are uniformly mixed together, the resultingmixture is put into a quartz crucible, where it is heated to 1250° C. soas to be roughly melted. At the time when substantially all theingredients are melted, the melt is taken out from the quartz crucibleand rapidly cooled so that a cullet is formed. This process is repeated.Then, thus prepared cullet is put into a platinum crucible and heated to1350° C. so as to be melted. This melting time is two hours. During twohours of melting, the melt is stirred three times with a platinum rod soas to be homogenized. After two hours have passed, the temperature isgradually lowered, and when the melt attains an appropriate viscosity,the melt is made to flow into a mold having a rod-shaped cavity. Thismelt, together with the mold, is put into an electric furnace, withinwhich the temperature has been set to 720° to 730° C. beforehand, so asto be subjected to an annealing process. Then, when the melt has apredetermined temperature within the electric furnace, the power of theelectric furnace is turned off so as to cool the melt naturally. Here, arod of the absorber glass is accomplished. Thereafter, the rod of theabsorber glass is taken out from the mold, shaven and polished to apredetermined size, and then processed into a glass fiber for FOP.

Initially, the inventors selected the colorant oxides as follows. Here,the object of the present invention is to attain a favorable absorptioncharacteristic in a broad band ranging from ultraviolet region throughvisible region to near infrared region and, in particular, to favorablymaintain the absorption in visible region while improving the absorptioncharacteristic in near infrared region. As ingredients for providing afavorable absorption characteristic in near infrared region, Cu²⁺ andFe²⁺ have been well-known. Taking into consideration that the oxidesincluding these ingredients should be contained in the absorber glass, acomposition which can further provide a favorable absorptioncharacteristic in ultraviolet to visible regions has been searched. Inorder to attain an absorbing capacity in a broad band, the compositionhas been made to contain at least FeO and Fe₂ O₃ as colorant oxides.Also, the content of the colorant oxides has been increased as much aspossible in order to attain a high absorbing capacity. Here, the searchhas been conducted while the fact that the combination and content ofthe colorant oxides may influence the degree of vitrification is takeninto account.

Initially, an absorber glass composed of 19% of SiO₂, 6% of B₂ O₃, 16%of La₂ O₃, 36% of BaO, 8% of TiO₂, 2.4% of FeO, 5.6% of Fe₂ O₃, 1% ofCo₂ O₃, and 6% of CuO was prepared according to the above-mentionedmethod. When the transmittance of this absorber glass (referred to as"EMA" hereinafter) at 790 nm was measured, it was 0.5% for the EMAhaving a thickness of 250 μm. Namely, it was elucidated that this EMAhad a quite favorable absorbing capacity in near infrared region.

When this EMA was drawn together with core and clad portions so as toprepare an FOP, however, the EMA was crystallized. Further testsperformed with varied temperature conditions for this drawing step haverevealed that the temperature condition under which the EMA is notcrystallized is quite limited, namely, it has been clarified that thetemperature control at the drawing step is quite difficult. Further,Cu²⁺ in the EMA easily diffused into the core, thereby remarkablycoloring the core. It has been elucidated that the spectraltransmittance of this FOP greatly decreases in the visible to nearinfrared region, thereby making the FOP practically unusable.Accordingly, it has been decided that the EMA of the present inventionshould contain no Cu ions.

Then, with respect to the above composition, the EMA containing about 5to 6% of conventional colorants instead of 6% of CuO was prepared, andits vitrification and absorption in the vicinity of 790 nm wereevaluated. As the colorant oxide used in place of CuO; Ni₂ O₃, MnO₂, Co₂O₃, SnO, V₂ O₅, WO₃, and Cr₂ O₃ were individually used. As a result, ithas been elucidated that vitrification is insufficient when MnO₂, SnO,V₂ O₅, WO₃, or Cr₂ O₃ is used, whereas vitrification is sufficientlyachieved when about 5 to 6% of Ni₂ O₃ or Co₂ O₃ is used.

In view of the foregoing results, studies have been conducted in orderto optimize the composition. Namely, 12 kinds of EMAs having differentcompositions were prepared. Then, the transmittance of each EMA withinthe wavelength range from about 300 nm to about 1,000 nm was measured.The respective compositions of these 12 kinds of glasses are shown inthe following Tables 1 and 2. All these glasses were favorablyvitrified.

                  TABLE 1                                                         ______________________________________                                        Unit: weight %                                                                Sample No.                                                                             40      57      58    59    60    61                                 ______________________________________                                        SiO.sub.2                                                                              18.1    21.3    24.8  28.0  30.9  33.6                               B.sub.2 O.sub.2                                                                        5.7     5.6     5.3   5.1   4.9   4.7                                La.sub.2 O.sub.3                                                                       17.1    8.3     8.0   7.6   7.3   7.0                                BaO      34.3    33.3    31.9  30.5  29.3  28.1                               TiO.sub.2                                                                              7.6     7.4     7.1   6.8   6.5   6.3                                FeO      3.4     4.4     4.3   4.1   3.9   3.8                                Fe.sub.2 O.sub.3                                                                       8.0     10.4    9.9   9.5   9.1   8.7                                Ni.sub.2 O.sub.3                                                                       4.8     0       0     0     0     0                                  Co.sub.2 O.sub.3                                                                       1.0     9.3     8.8   8.5   8.1   7.8                                MnO.sub.2                                                                              0       0       0     0     0     0                                  CuO      0       0       0     0     0     0                                  Transmittance                                                                          34      17.5    12.5  18    19    23                                 of       (%)                                                                  100-μm thick                                                               plate                                                                         (790 nm)                                                                      <TMA>                                                                         α(× 10.sup.7 ° C.sup.-1)                                            --      69.9    67.4  65.9  68.4  78.4                               Tg(°C.)                                                                         --      629     621   616   647   617                                At(°C.)                                                                         --      710     704   704   680   685                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Unit: weight %                                                                Sample No.                                                                             70      71      75    76    74    80                                 ______________________________________                                        SiO.sub.2                                                                              29.1    29.3    29.1  29.1  29.1  30.1                               B.sub.2 O.sub.3                                                                        0       0       0     0     0     0                                  La.sub.2 O.sub.3                                                                       7.7     10.3    7.7   7.7   7.7   8.9                                BaO      30.8    31.0    30.8  30.8  30.8  31.9                               TiO.sub.2                                                                              10.3    6.9     10.3  10.3  10.3  7.1                                FeO      4.1     4.1     0     22.2  6.7   6.9                                Fe.sub.2 O.sub.3                                                                       9.6     9.7     22.2  0.    15.5  16.1                               Ni.sub.2 O.sub.3                                                                       0       0       0     0     0     0                                  Co.sub.2 O.sub.3                                                                       8.5     8.6     0     0     0     0                                  MnO.sub.2                                                                              0       0       0     0     0     0                                  CuO      0       0       0     0     0     0                                  Transmittance                                                                          12.5    19      0.77  1.7   0.98  0.77                               of       (%)                                                                  100-μm thick                                                               plate                                                                         (790 nm)                                                                      <TMA>                                                                         α(× 10.sup.-7 ° C.sup.-1)                                           70.9    66.9    78.9  77.9  73.9  70.3                               Tg(°C.)                                                                         656     654     661   657   655   652                                At(°C.)                                                                         717     718     719   723   733   717                                ______________________________________                                    

The spectral transmittance of each of thus prepared EMAs with 12 kindsof compositions in a broad wavelength band extending from 300 to 1,000nm was measured, whereby the absorption characteristic thereof wasevaluated. It was confirmed that all these EMAs had a relativelyfavorable absorbing capacity throughout the wavelength range from 300 to1,000 nm. Tables 1 and 2 also show the transmittance of each EMA at awavelength of 790 nm which is in near infrared region.

First, while sample No. 40 containing Ni₂ O₃ exhibited the highesttransmittance (i.e., lowest absorptivity) at the wavelength of 790 nmamong the samples, its level is not unpractical yet.

Next, among sample Nos. 57 to 61 which include, instead of Ni₂ O₃, Co₂O₃ as a colorant oxide together with FeO and Fe₂ O₃, sample No. 58containing the highest total of contents of FeO and Fe₂ O₃ exhibited thelowest transmittance.

Also, in sample Nos. 70 and 71 which do not contain B₂ O₃ and whosetotal of contents of FeO and Fe₂ O₃ is nearly the same level as that ofsample No. 58, the glass transition point (Tg) and yield point (At) wereimproved.

Further, in sample Nos. 75 and 76 in which 22.2% of FeO or Fe₂ O₃ wasindividually used, transmittance was remarkably improved.

In view of these results, it has been elucidated that, when the contentsof FeO and Fe₂ O₃ are increased, a sufficiently high absorptivity can beobtained without other kinds of colorant oxides.

Based on this point of view, while both FeO and Fe₂ O₃ were used,studies were made to increase their contents. As a result, sample Nos.74 and 80 shown in Table 2 were prepared. Both of these samplesexhibited a remarkably favorable absorbing capacity at the wavelength of790 nm.

In samples 74 to 76 and 80 in which transmittance was remarkablyimproved, 18 to 40% by weight of SiO₂ is contained while the total ofcontents of FeO and Fe₂ O₃ is not smaller than 20% by weight.

The transmittances of sample Nos. 74 and 80 were further investigated indetail. Here, for comparison, a conventional EMA composed of 19% ofSiO₂, 6% of B₂ O₃, 16% of La₂ O₃, 36% of BaO, 8% of TiO₂, 2.4% of FeO,5.6% of Fe₂ O₃, and 1% of Co₂ O₃ was used as a conventional productwhich did not conform to the present invention. First, their changes intransmittance with respect to thickness are shown in FIG. 3 togetherwith those of sample Nos. 58 and 70 whose results were relativelyfavorable. As can be seen from FIG. 3, as compared with the conventionalproduct, sample Nos. 58, 70, 74, and 80 exhibited favorable absorptioncharacteristics at the wavelength of 790 nm and, in particular, sampleNos. 74 and 80 exhibited remarkably favorable results.

Next, FIG. 4 shows spectral transmittances of sample Nos. 74 and 80 andthe conventional product when their thickness is 130 μm. As can be seenfrom FIG. 4, as compared with the conventional product, sample Nos. 74and 80 exhibited remarkably favorable absorption characteristicsthroughout the whole wavelength region.

Further, the EMA of sample No. 80 and the EMA of the conventionalproduct were used together with known core and clad to preparerespective FOPs having a numerical aperture (N.A.) of 1, and thespectral transmittance of each FOP with a thickness of 3 mm wasmeasured. FIG. 5 shows thus measured spectral transmittances. As can beseen from FIG. 5, in the FOP using the EMA of the conventional product,the transmittance rises in near infrared region so that there is plentyof stray light and the effect of the absorber is small. In the FOP usingthe EMA of the present invention (sample No. 80), by contrast, it can beseen that there is no rise of transmittance in near infrared region,whereby the light is absorbed efficiently. Since this FOP has anumerical aperture (N.A.) of 1, in the case of diffused light, the wholelight component which has entered the core portion repeats totalreflection within the fiber and then exits from the opposite side of thesame fiber. The other light component which has entered the clad andabsorber portions, on the other hand, when the capacity of the absorberis incomplete, repeats refraction and is emitted therefrom at anidentical angle while being attenuated. Accordingly, these stray lightcomponents are added to the image transmitted by the core portion of theFOP, thereby deteriorating the resolution of the transmitted image.Since the FOP in accordance with the present invention can greatlyabsorb stray light in near infrared region while similarly absorbingstray light throughout ultraviolet to visible region, it can transmitimages while maintaining a favorable resolution in a broad range.

As explained in the foregoing, since the absorber glass of the presentinvention can greatly absorb stray light in near infrared region whilesimilarly absorbing stray light throughout ultraviolet to visibleregion, it can transmit images while maintaining a favorable resolutionin a broad range.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

The basic Japanese Application No. 168711/1995 filed on Jul. 4, 1995 ishereby incorporated by reference.

What is claimed is:
 1. A glass comprising in % by weight:18 to 40% ofSiO₂ ; 25 to 40% of BaO; 4 to 20% of La₂ O₃ ; 3 to 30% of FeO; and 3 to30% of Fe₂ O₃.
 2. A glass according to claim 1, further comprising notgreater than 10% by weight of B₂ O₃.
 3. A glass according to claim 1,further comprising not greater than 15% by weight of TiO₂.
 4. A glassaccording to claim 1, further comprising not greater than 15% by weightof Co₂ O₃.
 5. A glass according to claim 1, wherein said glasssatisfies, in % by weight, the following relationship:

    (content of SiO.sub.2)+(content of FeO)+(Content of Fe.sub.2 O.sub.3)>35%.


6. A glass according to claim 1, further comprising 3 to 6% by weight ofNi₂ O₃.
 7. A glass according to claim 1, wherein said glass issubstantially free of CoO and Co₂ O₃.
 8. A glass comprising in % byweight:20 to 35% of SiO₂ ; 5 to 12% of TiO₂ ; 25 to 35% of BaO; 7 to 10%of La₂ O₃ ; 3 to 9% of FeO; and 7 to 21% of Fe₂ O₃.
 9. A glass accordingto claim 8, further comprising not greater than 6% by weight of B₂ O₃.10. A glass according to claim 8, further comprising not greater than10% by weight of Co₂ O₃.
 11. A glass according to claim 8, wherein saidglass is substantially free of CoO and Co₂ O₃.
 12. A glass comprising in% by weight:23 to 31% of SiO₂ ; 7 to 11% of TiO₂ ; 30 to 32% of BaO; 7to 8% of La₂ O₃ ; 3 to 5% of FeO; and 8 to 12% of Fe₂ O₃, wherein saidglass is substantially free of CoO and Co₂ O₃.
 13. A glass according toclaim 12, further comprising not greater than 6% by weight of B₂ O₃. 14.A glass comprising in % by weight:28 to 31% of SiO₂ ; 7 to 11% of TiO₂ ;30 to 32% of BaO; 7 to 8% of La₂ O₃ ; 6 to 9% of FeO; and 15 to 21% ofFe₂ O₃,wherein said glass is substantially free of CoO and Co₂ O₃.
 15. Aglass according to claim 14, wherein said glass is substantially free ofB₂ O₃.